1. Field of the Invention
The present invention relates to a water-jet propulsion personal watercraft (PWC). More particularly, the present invention relates to a personal watercraft in which a V-type four-cycle engine is mounted.
2. Description of the Related Art
In recent years, jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The jet-propulsion personal watercraft include a straddle-type personal watercraft equipped with a seat mounted over an upper portion of a body and straddled by an operator, and a stand-up type personal watercraft provided with a foot deck formed on a rear portion of the body on which an operator rides in a standing position. The stand-up type personal watercraft is relatively small. The straddle-type personal watercraft can accommodate three or more, or two or fewer persons.
In both the stand-up type personal watercraft and the straddle-type personal watercraft, the body typically includes a hull and a deck covering the deck from above, and a deck opening is formed on an upper surface of the deck so that an engine and auxiliary devices may be mounted into the body therefrom. The engine is mounted within a space surrounded by a hull and a deck and located below the deck opening. A water jet pump is equipped on a rear portion of the body. Driven by the engine, the water jet pump pressurizes and accelerates water sucked from a water intake generally provided on a bottom surface of the hull and ejects it rearward from an outlet port of the water jet pump. As the resulting reaction, the personal watercraft is propelled forward.
The engine may be generally categorized as an in-line engine, or a V-type engine, etc., according to the arrangement of cylinders, and may be categorized as a two-cycle engine, or a four-cycle engine, etc., according to combustion stroke. The engine is mounted within the body in such a manner that a crankshaft extends in parallel with a pump shaft of the water jet pump, i.e., in a longitudinal direction of the body, or the crankshaft extends in a width direction of the body, i.e., in a lateral direction of the body.
Recently, in some personal watercraft, a four-cycle in-line engine having a crankshaft extending in the longitudinal direction has been put into practical use instead of the conventional two-cycle in-line engine, as the engine configured to drive the water jet pump of the personal watercraft. In other personal watercraft, a four-cycle in-line engine has a crankshaft extending in the lateral direction, or a V-type engine has a crankshaft extending in the longitudinal direction (see Japanese Laid-Open Patent Application Publication No. 11-208582, and U.S. Pat. No. 5,853,308). In the V-type engine having the crankshaft extending in the longitudinal direction of the body, adjacent cylinders are arranged in V-shape as seen in a rear view.
Since the engine is generally a heavy component in the personal watercraft, its center of gravity affects the attitude of watercraft. It is therefore desirable to locate the center of gravity of the engine mounted in the watercraft as low as possible. Nonetheless, since the conventional four-cycle in-line engine is constructed such that the cylinders extend substantially vertically, and a cam, a camshaft, air-intake and exhaust valves, which are relatively heavy, are located above the cylinders, the center of gravity tends to be high regardless of the placement of the crankshaft.
On the other hand, since the V-type four-cycle engine has inclined cylinders, its center of gravity is located relatively low in contrast to the in-line engine having the cylinders extending substantially vertically. The V-type four-cycle engine can be designed to reduce a dimension in an axial direction of the crankshaft. In addition, the V-type four-cycle engine can smoothly rotate by inhibiting its vibration caused by reciprocation of pistons.
In the case of the stand-up type personal watercraft, the body has a relatively small width and a narrow internal space. In addition, for the purpose of rigidity of the body, a deck opening is designed to have a limited opening area, and hence a small dimension in the width direction. On the other hand, the V-type four-cycle engine has a relatively large dimension in the direction perpendicular to the crankshaft, i.e., a dimension of the engine in the width direction of the body with the crankshaft extending in the longitudinal direction. Therefore, it is difficult to mount the V-type four-cycle engine into the body through the deck opening.
If such a V-type engine is mounted within the body such that the crankshaft extends in the longitudinal direction, cylinder heads located above the cylinders arranged in a V-shape extend partially outside the deck opening within the body. In this structure, a valve drive system contained within the cylinder heads is difficult to maintain through the deck opening. In some V-type engines, auxiliary devices such as an exhaust manifold and an oil tank are arranged below the inclined cylinders (i.e., in the vicinity of the bottom of the body). If this V-type engine is mounted within the body such that the crankshaft extends in the longitudinal direction, the auxiliary devices located in the vicinity of the bottom portion of the body is difficult to maintain through the deck opening, because a space between the engine and an inner wall of the body is small.